Elevated distal flow rates resulting from volume expansion or diuretic therapy increase the delivery of fluid and electrolytes to the distal nephron resulting in stimulation of K+ secretion, often reducing plasma [K+] to critically low levels. It is known that "K+ wasting" is due partially to high flow rates that stimulate K+ secretion in the distal nephron. However, the cellular mechanisms for flow-stimulated K+ secretion have never been understood. In this proposal, we plan to address this question using novel molecular knockdown and gene transduction strategies in combination with traditional in vivo and in vitro methods for analyzing K+ secretion and channel activity. In the mammalian kidney, K+ is secreted in the distal nephron, which includes the connecting tubules and principal cells (PC) of cortical collecting ducts (CCD) and is primarily mediated under basal conditions by inward rectifying K+ channels (Kir1.1). However, recent studies have shown that large Ca-activated K+ channels (BK) are the pathways for flow-stimulated K+ secretion. We found that the BK-B1 accessory subunit, known to heighten Ca sensitivity and confer cGMP-kinase activation of the pore-forming BK-a, was expressed specifically in the rabbit and mouse connecting tubules (CNT). Moreover, we found that BK-B1 mice (unlike the wild type controls) failed to increase K+ excretion in response to volume expansion (VE)-evoked increases in flow rate. Flow-dependent K+ secretion has been demonstrated in the isolated rabbit CNT suggesting an intrinsic mechanism, such as stretch or Ca-mediated activation of BK. However, when examined with in vivo micropuncture experiments, the relation for distal flow rate vs. rate of K+ secretion has an increased slope, suggesting that other, extrinsic mediators, are influencing K+ secretion from outside the CNT. 1 possible mediator is nitric oxide, which is released with increased flow rates in the thick ascending limb and has been implicated in the increase in flow-induced K excretion in dogs. Therefore, it is hypothesized that the BK-B1 subunit augments K+ secretion in response to flow rate in the mammalian connecting tubule by conferring Ca and/or NO/cGMP sensitivity to BK-B1. The hypothesis of this proposal is based on 3 recent key findings from this laboratory: 1. BK-B1 knockout mice do not demonstrate flow-dependent K+ secretion. 2. The BK-P1 subunit is expressed specifically in the apical membranes of mammalian connecting tubules. 3. The presence of the BK-B1 enhances the calcium sensitivity of BK and is necessary for activation of BK by cGMP-mediated pathways. We will examine this hypothesis with the following Specific Aims: 1. Use BK-B1 knockout mice to determine if the BK-01 subunit is necessary for flow-dependent n secretion. 2. Determine the localization of the BK-B1 subunit in the distal nephron. 3. Determine by gene transduction whether the BK-beta1 in the mouse connecting tubule is necessary for flow-induced K+ secretion. 4. Determine the role of NO in the flow-induced increase in K+ secretion in the mouse distal nephron. Knockout mice to determine if the BK-/31 subunit is necessary for flow-dependent K+ secretion.